The present invention relates to a process for rounding off granular particles of solid material, in particular granulated hard materials.
Rounded particles are already produced by means of various processes. A widely practiced method for example is the spraying of molten material in a gas stream. This process is employed mainly to produce spherical shaped metal powders, but is also used to make powders of refractory material. The process has the disadvantage, however, that it cannot be employed widely for hard materials, that is, oxides, carbides, borides and nitrides or refractory materials which have a hardness .gtoreq.7 on the Mohs hardness scale, the reason being that many of these materials have very high melting points and not all remain chemically stable in the molten state. Also, it is not always possible to prevent hollow spherical particles from forming during spraying.
Another known method for producing rounded particles is that of surface melting and thereby rounding off the particles in a high energy stream such as a plasma stream. Again, this process can be employed only for materials which are stable in the molten state, and is furthermore limited to particle sizes of about 100-200 .mu.m.
Rounded solid bodies can, according to the GB Pat. Publ. No. 2 037 727, be produced by agglomeration or compaction of suitably fine powders followed by sintering. Such a process is also disadvantageous as the material must first be ground down to about 1/100 to 1/1000 of the size of the final, desired particle in order that a sinterable powder results. Also, the range of diameter of the final product is limited to about 0.4-5 mm.
Other processes which come into question, such as the sol-gel process and spray granulation, also suffer from disadvantages. The sol-gel process can be employed for only certain materials, and is used mainly for producing oxide spheres in the range .ltoreq.500 .mu.m. The quality of the product from the spray process is inadequate. Usually only particles of low density can be made this way, due to the loose structure.
The object of the present invention is therefore to create a process for rounding off particulate materials of any particle shape, in particular hard, granulated materials, which does not exhibit the disadvantages of the above mentioned processes.